Ohm's Law Calculator

How the Ohm's Law Calculator works

The Ohm's Law calculator solves for voltage, current, resistance, or power when any two of the four electrical quantities are known. It covers all four forms of Ohm's Law (V = IR, I = V/R, R = V/I) and the power triangle (P = IV, P = V²/R, P = I²R). Electronics students, hobbyists, and professional engineers use it daily for circuit design, component selection, and troubleshooting.

The four Ohm's Law relationships

Georg Ohm's 1827 law states that V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. From this single equation, three more are derived: I = V/R, R = V/I. Power adds two more dimensions: P = V × I, which combines with Ohm's Law to give P = V²/R and P = I²R. The calculator covers all six derived forms so any two known quantities yield all four unknowns.

Series and parallel circuit analysis

In a series circuit, total resistance is the sum of individual resistances (Rt = R1 + R2 + R3), and the same current flows through every component. In a parallel circuit, conductances add (1/Rt = 1/R1 + 1/R2), and voltage is the same across every branch. The calculator handles both configurations: enter total resistance for the full-circuit solution, or use the tool repeatedly for individual branch analysis.

Practical troubleshooting applications

A common use case is selecting a current-limiting resistor for an LED: if an LED has a forward voltage of 2 V and needs 20 mA on a 5 V supply, R = (5 − 2) / 0.02 = 150 Ω. Another is checking whether a wire is adequate: at 10 A through 0.1 Ω wire resistance, the voltage drop is 1 V and power dissipated is 10 W — the Ohm's Law calculator makes these checks instantaneous.

Unit prefixes and range handling

Practical circuits span an enormous range of values: from milliohms in PCB traces to megaohms in CMOS inputs, from microamps in sensor bias currents to tens of amperes in motor drives. The calculator supports standard SI prefixes (mΩ, kΩ, MΩ, mA, µA) so you can enter values in the natural engineering units for your application without converting to base units manually.

Frequently asked questions

What is Ohm's Law?

Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. It is the foundational relationship for all basic circuit analysis and component selection.

How is power calculated?

Electrical power (P) represents the rate at which energy is consumed or dissipated. It can be calculated using three equivalent formulas: P = V × I (voltage times current), P = I² × R (current squared times resistance), or P = V² / R (voltage squared divided by resistance). The result is in watts. This is the Joule heating formula — the power dissipated as heat in a resistor.

Can I calculate power with just two values?

Yes. With any two of the four quantities — voltage (V), current (I), resistance (R), and power (P) — you can calculate the remaining two using Ohm's Law and the power formulas. This calculator accepts any two inputs and solves for all four values automatically, showing results for V, I, R, and P simultaneously.